① CounterCell数组不为空，优先利用数组中的CounterCell记录数量
② 如果数组为空，尝试对baseCount进行累加，失败后，会执行fullAddCount逻辑
③ 如果是添加元素操作，会继续判断是否需要扩容
private final void addCount(long x, int check) { CounterCell[] as; long b, s; //当CounterCell数组不为空，则优先利用数组中的CounterCell记录数量 //或者当baseCount的累加操作失败，会利用数组中的CounterCell记录数量 if ((as = counterCells) != null || !U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) { CounterCell a; long v; int m; //标识是否有多线程竞争 boolean uncontended = true; //当as数组为空 //或者当as长度为0 //或者当前线程对应的as数组桶位的元素为空 //或者当前线程对应的as数组桶位不为空，但是累加失败 if (as == null || (m = as.length - 1) < 0 || (a = as[ThreadLocalRandom.getProbe() & m]) == null || !(uncontended = U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) { //以上任何一种情况成立，都会进入该方法，传入的uncontended是false fullAddCount(x, uncontended); return; } if (check <= 1) return; //计算元素个数 s = sumCount(); } if (check >= 0) { Node<K,V>[] tab, nt; int n, sc; //当元素个数达到扩容阈值 //并且数组不为空 //并且数组长度小于限定的最大值 //满足以上所有条件，执行扩容 while (s >= (long)(sc = sizeCtl) && (tab = table) != null && (n = tab.length) < MAXIMUM_CAPACITY) { //这个是一个很大的正数 int rs = resizeStamp(n); //sc小于0，说明有线程正在扩容，那么会协助扩容 if (sc < 0) { //扩容结束或者扩容线程数达到最大值或者扩容后的数组为null或者没有更多的桶位需要转移，结束操作 if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 || sc == rs + MAX_RESIZERS || (nt = nextTable) == null || transferIndex <= 0) break; //扩容线程加1，成功后，进行协助扩容操作 if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) //协助扩容，newTable不为null transfer(tab, nt); } //没有其他线程在进行扩容，达到扩容阈值后，给sizeCtl赋了一个很大的负数 //1+1=2 --》 代表此时有一个线程在扩容 //rs << RESIZE_STAMP_SHIFT)是一个很大的负数 else if (U.compareAndSwapInt(this, SIZECTL, sc, (rs << RESIZE_STAMP_SHIFT) + 2)) //扩容，newTable为null transfer(tab, null); s = sumCount(); } } } 2.5.2 fullAddCount方法① 当CounterCell数组不为空，优先对CounterCell数组中的CounterCell的value累加
② 当CounterCell数组为空，会去创建CounterCell数组，默认长度为2，并对数组中的CounterCell的value累加
③ 当数组为空，并且此时有别的线程正在创建数组，那么尝试对baseCount做累加，成功即返回，否则自旋
private final void fullAddCount(long x, boolean wasUncontended) { int h; //获取当前线程的hash值 if ((h = ThreadLocalRandom.getProbe()) == 0) { ThreadLocalRandom.localInit(); // force initialization h = ThreadLocalRandom.getProbe(); wasUncontended = true; } //标识是否有冲突，如果最后一个桶不是null，那么为true boolean collide = false; // True if last slot nonempty for (;;) { CounterCell[] as; CounterCell a; int n; long v; //数组不为空，优先对数组中CouterCell的value累加 if ((as = counterCells) != null && (n = as.length) > 0) { //线程对应的桶位为null if ((a = as[(n - 1) & h]) == null) { if (cellsBusy == 0) { // Try to attach new Cell //创建CounterCell对象 CounterCell r = new CounterCell(x); // Optimistic create //利用CAS修改cellBusy状态为1，成功则将刚才创建的CounterCell对象放入数组中 if (cellsBusy == 0 && U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) { boolean created = false; try { // Recheck under lock CounterCell[] rs; int m, j; //桶位为空， 将CounterCell对象放入数组 if ((rs = counterCells) != null && (m = rs.length) > 0 && rs[j = (m - 1) & h] == null) { rs[j] = r; //表示放入成功 created = true; } } finally { cellsBusy = 0; } if (created) //成功退出循环 break; //桶位已经被别的线程放置了已给CounterCell对象，继续循环 continue; // Slot is now non-empty } } collide = false; } //桶位不为空，重新计算线程hash值，然后继续循环 else if (!wasUncontended) // CAS already known to fail wasUncontended = true; // Continue after rehash //重新计算了hash值后，对应的桶位依然不为空，对value累加 //成功则结束循环 //失败则继续下面判断 else if (U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x)) break; //数组被别的线程改变了，或者数组长度超过了可用cpu大小，重新计算线程hash值，否则继续下一个判断 else if (counterCells != as || n >= NCPU) collide = false; // At max size or stale //当没有冲突，修改为有冲突，并重新计算线程hash，继续循环 else if (!collide) collide = true; //如果CounterCell的数组长度没有超过cpu核数，对数组进行两倍扩容 //并继续循环 else if (cellsBusy == 0 && U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) { try { if (counterCells == as) {// Expand table unless stale CounterCell[] rs = new CounterCell[n << 1]; for (int i = 0; i < n; ++i) rs[i] = as[i]; counterCells = rs; } } finally { cellsBusy = 0; } collide = false; continue; // Retry with expanded table } h = ThreadLocalRandom.advanceProbe(h); } //CounterCell数组为空，并且没有线程在创建数组，修改标记，并创建数组 else if (cellsBusy == 0 && counterCells == as && U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) { boolean init = false; try { // Initialize table if (counterCells == as) { CounterCell[] rs = new CounterCell[2]; rs[h & 1] = new CounterCell(x); counterCells = rs; init = true; } } finally { cellsBusy = 0; } if (init) break; } //数组为空，并且有别的线程在创建数组，那么尝试对baseCount做累加，成功就退出循环，失败就继续循环 else if (U.compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x)) break; // Fall back on using base } }图解
fullAddCount方法中，当as数组不为空的逻辑图解